The present invention relates to refrigeration systems that utilize at least one evaporator and provides for continuous refrigerant flow during shut off of evaporated airflow.
Most vehicle air conditioning systems in use today utilize a suction accumulator to store and release refrigerant as required by operating conditions, and to separate incoming gas and liquid mixture. Known vehicle air conditioning systems include a compressor that is engine driven or driven by an electric motor, at least one evaporator and a condenser operatively connected together within the system. A mechanism to control refrigerant flow is typically provided between the evaporator and condensor to insure that the system will effectively operate under variable vehicle speed and load conditions. Known examples of such control mechanisms include a fixed capillary or orifice tube.
For those applications that have both a front and rear evaporator units, a fixed orifice tube expansion device is typically used with for a front evaporator unit while the rear evaporator unit utilizes a thermostatic expansion valve and flows the suction gas directly to the compressor inlet line. An example of such a system is shown in FIG. 1.
As such known systems, compressor lubrication oil is circulated with the system refrigerant during operation but a problem of oil trapping occurs when the rear unit blower is shut off (common mode). Even with a small bleed thru the rear thermostatic expansion valve (many use this) the oil separates in the rear evaporator and the low mounted rear suction line resulting in oil starvation to the compressor. Total shut-off of the rear liquid line does not solve the problem as refrigerant migration results in oil eventually ending up in the rear plumbing and coil.
If one were to simply dump liquid refrigerant into the suction accumulator from the rear evaporator at a rate greater than the oil bleed hole allows from the suction accumulator then in a short time a flow imbalance would occur wherein the front evaporator would progressively be starved since the condenser outflow would be greater than the inflow to the compressor, thus filling the accumulator. A new balance would result in severe starvation of the front evaporator and result in unsatisfactory air-cooling performance. This already occurs in production systems at certain high-speed high load conditions if adequate charge is trapped in the rear system with rear evaporator blower shut off. This problem is greatly aggravated with loss of some charge.
To aid the compressor durability of current dual systems, one approach has been to add a disproportionate large amount of oil to the system. However, this results in some deterioration of cooling performance due to more oil film on heat exchanger surfaces.
Accordingly, there is a need for an improved air conditioning system that does not require additional oil concentration and prevents front evaporator starvation during rear evaporator blower shut off conditions.
In one embodiment of the invention, an air conditioning system having dual evaporators solves the oil-trapping problem by allowing a continuous flow of rear evaporator refrigerant even with the rear evaporator airflow shut off. Essentially, continuous flow is achieved at an adequate rate to circulate oil to a suction accumulator where it can be returned to the compressor. Satisfactory front evaporator operation during this rear evaporator shut off is accomplished by use of an eductor assembly to circulate an adequate amount of refrigerant from the suction accumulator into an incoming feed line to the front evaporator to insure that the front evaporator is not starved. The rear evaporator may be provided with a fixed orifice tube, a variable orifice tube or a thermostatic expansion valve with an adequate bleed to return oil satisfactorily.
This invention also discloses suction accumulators for preventing oil trapping. In one embodiment, a U-tube is provided in the suction accumulator with an oil bleed hole being formed at the base of the U. An eductor conduit in communication with the suction accumulator directs liquid to an eductor assembly to feed the front evaporator unit. When the rear evaporator is shut off, the eductor assembly may also draw liquid from a rear suction line. Thus, reduced recirculation from the suction accumulator is achieved when the rear evaporator is operating to advantageously limit recirculation to prevent liquid-vapor separation problems in the suction accumulator and also limits the occurrence of oil dilution in the suction accumulator.
Alternative embodiments of a suction accumulator for use with a either a dual evaporator system or a single evaporator system are also disclosed. In one alternative embodiment, a baffle assembly serves to limit the liquid level above an oil bleed hole to prevent too much liquid bleed from the accumulator. In another alternative embodiment, two chambers are provided in the suction accumulator, each chamber having a baffle assembly.